Geology in space

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Some of the most spectacular images from within our Solar System come from Nasa’s Cassini spacecraft as it orbits Saturn. One of the reasons for this is that many of the vistas are dominated by the planet’s rings.

Saturn, its rings (and their shadow), and the moon Tethys (NASA/JPL/SSI)

The rings of Saturn are made up mainly of lumps of ice (with a small amount of dust), which while individually are only millimeters up to about a kilometer in size, add up to whole rings structures which whilst only a kilometer deep extend laterally for hundreds of kilometers.

There is still a lot of debate about how they formed and how long they will last. Cassini’s final orbits are designed to answer some of these questions by flying between the planet and the rings and we should get more answers in the months and years after the end of the mission once the data is analyzed. Current theories suggest that the rings formed around the same time as Saturn did; either from the same planetary nebula which formed the planet or from a moon which was torn apart by Saturn’s gravity. Other researchers have suggested that they are only 100 million years old and transient features which will eventually fall into the planet (over very long time periods). whilst the majority of the evidence points to them being very long lived so far. The exception to this is the outer E rings, these are known to have been generated by the ice volcanoes of Enceladus throwing out icy material into the orbit of Saturn and form a ring.

Rings are dynamic with transient features such as “spokes” of dust caught in the magnetic field chasing around jus above the surface of the rings, disturbances within them like propellers formed by the gravitational distortion of moonlets, or the ripples generated by Daphnis as it travels within a gap in the rings.

Daphnis a small moon causing ripples within the rings of Saturn (NASA/JPL/SSI)

Pan, a moonlet within a gap in the rings has made the gap by collecting up the ice particles, forming a band around its middle.

Pan, which orbits within the rings, has built up ice within its middle (NASA/JPL/SSI)

It is not just Saturn which has rings, Neptune, Uranus, and Jupiter all have rings and even the 250 km diameter small planetoid 10199 Chariklo has a ring system around it. Temporary rings probably occur on all planets as comets and asteroids get too close are torn apart by the gravitational pull. forming a ring of debris which would eventually fall to the surface, Mars may develop a transient ring system, when Phobos is torn apart by tidal forces as it slowly spirals towards the surface. This ring will be transient as the material itself falls onto the surface of Mars.

Infrared image of the rings of Jupiter

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Volcanoes on Earth have fascinated and terrified people for millennia, extinct volcanoes are seen all over the solar system; on Mars & Mercury and the Moon to name just a few places. Venus and Earth both have active volcanoes on their surface. However, the most volcanically active place in the solar system is Jupiter’s moon Io.

Io is kept volcanically active by the tidal forces of Jupiter and its moons pulling at it, the changes in gravity cause it to flex and bulge which generates heat inside it and causing parts of its to melt. If it was not for this tidal heating then it would no longer be volcanically active, being small and so cooling quickly.

Volcanism on Io, (NASA/JPL/UoA)

Whilst there are some other geological features on Io, such as mountains, there are few impact craters observed, which shows that it has a relatively young surface. It does have many volcanic craters. The volcanoes are dominated by basalt lavas (like those on Hawaii) these can form lava flows which flow over hundreds of kilometres. In addition to this, the volcanoes erupt lava flows of sulphur and sulphur dioxide, The crater-like depressions which are seen on the surface, look a lot like calderas. On Earth, which form when a magma chamber is emptied and a volcano collapse in on itself (it is not known if this same mechanism occurs on Io).

The first evidence of volcanic activity was plumes of material spotted rising above the surface like a fountain. These plumes create a huge amount of material mainly sulphur dioxide which rains back onto the surface.

The highly volcanically active nature of Io means that each year the equivalent of around 1-1.5 cm of material over the whole moon is produced, a staggeringly fast rate of depositing material over a large area in geological terms. This leads to craters on the surface being hidden giving it one of the youngest surfaces of all the bodies in the solar system.

Volcanic plume on Io as seen from Voyager (Nasa/JPL/USGS)

Volcanoes dominate much of the landscape of Io, and perhaps provide some insights into what was happing other bodies in the solar system before the cooled down and became volcanically inactive. It shows how the appearance bodies can be dominated by the external gravitation influence and interaction with of other bodies.